1. Field of Invention
The invention relates to a technique to perform a level conversion on multilevel input image data to produce multilevel output image data having fewer levels than the multilevel input image data.
2. Description of Related Art
Conventionally, error diffusion is known as one technique to convert multi-level input image data into multilevel output image data having fewer levels than multilevel input image data. In this technique, error values generated in a level conversion process are saved with respect to respective pixels of the input image data. The error values are collected based on the error diffusion matrix, and fed back to input level values of unprocessed pixels. By providing feedback of the error values, reproductivity of density levels as an entire image can be improved.
In addition to the error diffusion method, dithering is one of the techniques also frequently used to convert multilevel input image data into multilevel output image data. Dithering is a technique that determines an output level value by comparing an input level value and a threshold value in a dither matrix one-on-one. The dithering technique can also improve the processing speed.
Some techniques to solve defects of error diffusion and dithering have been proposed. For example, U.S. patent application Publication No. 2003/0072035 discloses that an error utilization factor K is determined based on characteristics of input image data and output conditions, such as a printing resolution. The error utilization factor K is multiplied by an error value collected in an error distribution matrix, and then added to the input level value.